1. Field of the Invention
The present invention relates to eye examining and testing instruments and more particularly such eye examining and testing instruments which are provided with a detection system for detecting the positional relation between the objective lens and the eye being examined and tested.
2. Description of the Prior Art
Eye examining and testing instrument are known, for example, which can utilize an eye fundus camera, an eye refractometer and a large ophthalmoscope. In carrying out picture-taking, observation or measurement with such instrument it is essential to correctly set the instrument relative to the eye being examined and tested. In particular, in case of the eye fundus camera, an inaccurate setting of the instrument regarding the working distance and alignment between the objective lens and the human eye causes various troubles such as flare of image due to mixing of a portion of fundus illuminating light reflected upon the cornea into the effective light reflected upon the fundus.
For a conventional fundus camera, adjustment of the working distance has to be made in the following manner:
The examiner initially aligns the optical axis of the objective lens with a human eye to be examined while observing the light illuminating the cornea. When the illumination light becomes correctly incident upon the pupil, an alignment of the optical axis is obtained. Thereafter, the examiner adjusts the working distance while observing through the eyepiece of the instrument whether there is any scattered light or reflected light visible. When no scattered or reflected light becomes visible, there is given the correct working distance. This operation of adjustment must be done repeatedly whenever any change in the positional relation occurs between the objective optical system and the human eye. This is very troublesome and time-consuming.
This problem is further enhanced in case of a non-dilatation type of eye fundus examining instrument. For this type of instrument, invisible light such as infrared rays is used in focusing and adjusting the visual field. Since the examiner can not observe the fundus illuminating light and other harmful lights with the naked eyes, it is impossible to employ the above described procedure of adjustment. Therefore, in case of the use of infrared rays, a higher skillfulness is required for adjustment and the frequency of incorrect setting of the instrument becomes higher.